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Transverse Spin Physics in pp Collisions at RHIC

Transverse Spin Physics in pp Collisions at RHIC. Carl A. Gagliardi Texas A&M University Outline. Introduction Forward rapidity measurements Mid-rapidity measurements Looking ahead. RHIC : the Relativistic Heavy Ion Collider. Search for and study the Quark-Gluon Plasma

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Transverse Spin Physics in pp Collisions at RHIC

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  1. Transverse Spin Physics in pp Collisions at RHIC Carl A. Gagliardi Texas A&M University Outline • Introduction • Forward rapidity measurements • Mid-rapidity measurements • Looking ahead

  2. RHIC: the Relativistic Heavy Ion Collider • Search for and study the Quark-Gluon Plasma • Explore the partonic structure of the proton • Determine the partonic structure of nuclei

  3. From PDFs to polarized PDFs Consider a proton moving toward the right • There are really three different sets of PDFs for the proton   q(x) g(x) Unpolarized PDF + Proton spin    “Polarized” distribution Δq(x) Δg(x) Proton spin  “Transversity” distribution δq(x)  

  4. Current knowledge of the polarized distributions • Quarks and antiquarks only carry ~30% of total proton spin • Proton “spin crisis” Anselmino et al, arXiv:0807.0173 DSSV, PRL 101, 072001 • Gluon and anti-quark polarizations have large uncertainties • Know very little about orbital motion • Transversity also has large uncertainties RHIC transverse spin program

  5. RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS PHOBOS Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin flipper Spin Rotators (longitudinal polarization) Solenoid Partial Siberian Snake Pol. H- Source Helical Partial Siberian Snake LINAC BOOSTER AGS Internal Polarimeter AGS 200 MeV Polarimeter AGS pC Polarimeters Strong Helical AGS Snake Rf Dipole RHIC: the world’s first polarized hadron collider • Spin varies from rf bucket to rf bucket (9.4 MHz) • Spin pattern changes from fill to fill • Spin rotators provide choice of spin orientation • “Billions” of spin reversals during a fill with little if any depolarization

  6. Definition: dσ↑(↓) – cross section for scattering to the left when incoming proton has spin up(down) Two methods of measurements: Single arm calorimeter: R – relative luminosity Pbeam – beam polarization Two arm (left-right) calorimeter: Less sensitive to instrumental effects Left π0, xF<0 π0, xF>0 p  p Right Transverse single-spin asymmetries Positive AN: more p0 going to left of the polarized beam

  7. Transverse single-spin asymmetries at forward rapidity • Large single-spin asymmetries at CM energy of 19.4 GeV FNAL E704 • Weren’t supposed to be there in naïve pQCD

  8. √s=23.3GeV √s=52.8GeV Data-pQCD differences at pT=1.5GeV NLO calculations with different scales: pT and pT/2 Ed3s/dp3[mb/GeV3] Ed3s/dp3[mb/GeV3] q=5o q=10o q=15o q=53o q=22o xF xF Forward π0 production at ISR energies Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at ISR energies Maybe the E704 results arise from soft physics?

  9. STAR First AN measurement at RHIC Similar to result from E704 experiment (√s=19.4 GeV, 0.5 < pT < 2.0 GeV/c) PRL 92, 171801 (2004) Can be described by several models: Sivers: spin and kT correlation in parton distribution functions (initial state) Collins: spin and kT correlation in fragmentation function (final state) Qiu and Sterman (initial state) / Koike (final state): twist-3 pQCD calculations, multi-parton correlations √s=200 GeV, <η> = 3.8

  10. STAR Forward pp  π0 + X cross sections at 200 GeV PRL 97, 152302 • The error bars are statistical plus point-to-point systematic • Consistent with NLO pQCD calculations at 3.3 < η < 4.0 • Data at low pT trend from KKP fragmentation functions toward Kretzer. NLO pQCD calculations by Vogelsang, et al.

  11. Sivers and Collins effects in pp collisions Sivers mechanism:initial-state kT dependence in the parton distribution Collins mechanism:final-state asymmetry in the forward jet fragmentation SP SP kT,q p p p p Sq kT,π Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity Observed transverse single-spin asymmetries could arise from the Sivers effect or Collins effect, or from a linear combination of the two

  12. Sivers Collins Sivers and Collins effects in deep-inelastic scattering • Semi-inclusive DIS can distinguish the Sivers and Collins effects • HERMES finds both are non-zero

  13. STAR Recent π0 results at 200 GeV from STAR PRL 101, 222001 • Sivers fit to HERMES SIDIS describes η = 3.3; overpredicts η = 3.7 • Twist-3 fit to E704 plus preliminary STAR Runs 3 and 5 data describes η = 3.7; underpredicts η = 3.3

  14. BRAHMS Charged pion measurements at 200 GeV from BRAHMS • Sign dependence of charged pion asymmetries seen in FNAL E704 persists to 200 GeV 4 deg (~3) 2.3 deg (~3.4)

  15. p+p0+X at s=62.4 GeV/c2 3.0<<4.0 0 results at 62.4 GeV from PHENIX p+p0+X at s=62.4 GeV/c2 • Asymmetries are comparable or larger at 62.4 GeV than they are at 200 GeV

  16. BRAHMS Charged-hadron results at 62.4 GeV from BRAHMS PRL 101, 042001 • Very large asymmetries! • K- (= su )asymmetry is a surprise. Sea-quark Sivers effect or disfavored fragmentation? Limitation of the BRAHMS measurements: Very strong correlation between xF and pTfrom small acceptance

  17. STAR Inclusive π0AN(pT) in xF bins PRL 101, 222001 • Combined data from three runs • at <η>=3.3, 3.7 and 4.0 • Measured AN is not a smooth • decreasing function of pT • as predicted by theoretical • models Kouvaris et al, PRD 74, 114013

  18. STAR STAR 2006 PRELIMINARY A potential fly in the ointment? • To date, the η meson has looked like a “high-mass, low-yield π0” in all measurements at RHIC • AN for the η mass region is much larger at high xF η ~ 3.66

  19. Mid-rapidity inclusive pion AN PRL 95, 202001 • Mid-rapidity pion yields are gluon-dominated at these pT • No Collins effect for gluons • May help to constrain the gluon Sivers function

  20. STAR Mid-rapidity inclusive jet AN -0.5< <0.0 -0.9< <-0.5 STAR Preliminary STAR Preliminary • Gluon-dominated at low pT • Dominated by qg scattering at higher pT • Within uncertainties, AN is consistent with zero PT (GeV/c) 0.5< <0.9 0.0< <0.5 PT (GeV/c) STAR Preliminary STAR Preliminary PT (GeV/c) PT (GeV/c)

  21. Sivers effect in di-jet production Sivers effect: Sivers ON spin 1 • Left/right asymmetry in the kT of the partons in a polarized proton • Spin dependent sideways boost to di-jets • Measure the di-jet opening angle as a function of proton spin • Requires parton orbital angular momentum  > 180 for kTx > 0 di-jet bisector kTx 2

  22. STAR Mid-rapidity di-jet Sivers effect measurementPRL 99, 142003 • Observed asymmetries are an order of magnitude smaller than seen in semi-inclusive DIS by HERMES • Detailed cancellations of initial vs. final state effects and u vs. d quark effects, coupled with very small gluon Sivers effect?

  23. Mid-rapidity di-hadron Sivers effect measurement • PHENIX is performing a similar measurement • Back-to-back correlations between a trigger π0 and an away-side charged hadron

  24. Separating Sivers and Collins effects in pp collisions Sivers mechanism:asymmetry in the forward jet or γ production Collins mechanism:asymmetry in the forward jet fragmentation SP SP kT,q p p p p Sq kT,π Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity • Need to go beyond inclusive hadrons to measurements of jets or direct γ

  25. Large-acceptance forward detectors PHENIX Muon Piston Calorimeters 3.1 < |η| < 3.7 STAR Forward Meson Spectrometer 2.5 < η < 4.0 • Both PHENIX and STAR have installed large-acceptance electromagnetic calorimeters in the forward direction

  26. First look at jet-like events with the STAR FMS • Comparisons of the jet profile and effective mass in data vs. PYTHIA + GEANT simulations

  27. Transverse spin forward g + mid-rapidity jet Bacchetta et al., PRL 99, 212002 • Conventional calculations predict the asymmetry to have the same sign in SIDIS and +jet • Calculations that account for the repulsive interactions between like color charges predict opposite sign • Critical test of our basic theoretical understanding

  28. In the further future: Drell-Yan • In SIDIS, final-state interaction of outgoing quark with proton remnant involves opposite color charges – attractive • In Drell-Yan, initial-state interaction of the incoming quark with the spectator components of the proton involves like color charges – repulsive • Sign of AN should reverse

  29. Conclusions • RHIC has observed large transverse single-spin asymmetries for forward particle production • These asymmetries may provide evidence for parton orbital angular momentum and/or quark transversity • Measurements to identify the underlying cause(s) are underway • Future measurements will provide a direct illustration of attractive vs. repulsive color-charge interactions • RHIC, the world’s first polarized hadron collider, is generating a wealth of new data regarding the spin structure of the proton • Stay tuned!

  30. Low-pT forward transverse single-spin asymmetries STAR BBCs • Rotator tuning for longitudinal polarization requires local polarimetry PHENIX ZDCs

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